TMS assessed cortical representation scales with stimulation intensity and muscle activation

Abstract

Transcranial magnetic stimulation (TMS) is routinely used to construct a map of corticospinal excitability (CSE). TMS elicited motor evoked potentials (MEPs) are known to increase both with stimulation intensity and muscle activation. Whilst a wide variety of stimulation intensities and levels of muscle activation are used to generate TMS maps, their effect on the cortical representation has yet to be systematically explored. Two experiments were performed to describe the effect of stimulation intensities (Experiment 1) and muscle activation (Experiment 2) on the map outcome measures: aspect ratio, centre of gravity (COG), map area and map volume. Twelve participants were recruited for each experiment. TMS maps were acquired from the first dorsal interosseous (FDI). Maps were acquired using 80 stimuli pseudorandomly across a 6x6 cm area with a 1.5 s interstimulus interval, allowing the maps to be acquired in two minutes. In Experiment 1 maps were compared at 5, 10, 20 and 40% of the maximum voluntary contraction. All maps were acquired with a stimulation intensity of 120% of the resting motor threshold (RMT). In Experiment 2 maps were compared at the stimulation intensities of 110, 120 and 130% of RMT, whilst the muscle was at rest. A significant increase in map area and map volume were observed with stimulation intensity and level of muscle activation as would be expected. Neither the COG nor the aspect ratio were changed with either increased stimulation intensity or muscle activation. This study demonstrates that the cortical representation scales with stimulation intensity and level of muscle activation, but the shape of the map does not change.